This invention relates to a helium gas liquefying apparatus which produces liquefied helium gas by introducing helium gas stock and suitably cooling the gas.
A conventional helium gas liquefying apparatus of this type has a structure as shown in FIG. 1. In the structure, a helium gas bomb 1, a compressor 2, a cooler 3 and a liquefied helium reservoir 4 have been connected by piping with a J-T valve 5 (which performed Joule-Thompson's effect), a return valve 6 and control valves 7 and 8. The following preliminary various operations have been carried out before liquefied helium gas (LHe) was produced in the reservoir 4.
The valves 5 and 5 are closed, the compressor 2 is then operated, helium gas (GHe) is introduced from the bomb 1 into the compressor 2, and the GHe compressed by the compressor 2 is then fed to the cooler 3. The cooler 3 includes a plurality of heat exhangers 9.sub.1, 9.sub.2, 9.sub.3, 9.sub.4, 9.sub.5 and expansion engines 10.sub.1, 10.sub.2 known per se. A series liquefying line 11 and a series return line 12 with the respective heat exchangers are provided in parallel with one another via a reverse flow heat exchanging arrangement. When the compressed GHe is introduced from the inlet 11' of the line 11 into the cooler 3, the expansion engines 10.sub.1, 10.sub.2 are respectively connected in parallel with the second and fourth heat exchangers 9.sub.2 and 9.sub.4 between the lines 11 and 12, and the GHe exhausted from the line 11 of the first heat exchanger 9.sub.1 is branched to the first expansion engine 10.sub.1, is expanded in the engine 10.sub.1, and the GHe which is thus lowered at its temperature via the expansion engine 10.sub.1 is sequentially passed through the line 12 of the second and first heat exchangers 9.sub.2 and 9.sub.1 and is returned to the inlet of the compressor 2 circularly. Thus, the GHe is gradually cooled via the first and second heat exchangers 9.sub.1, 9.sub.2.
Similarly, the second expansion engine 10.sub.2 cools the GHe branched from the third heat exchanger 9.sub.3, and the GHE is returned sequentially through the fourth, third, second and first heat exchangers 9.sub.4, 9.sub.3, 9.sub.2, 9.sub.1 to the compressor 2. In this manner, the GHe is progressively cooled even via the circulating line and the first preliminary operation for cooling the GHe is carried out by circulating the GHe to the fourth heat exhanger 9.sub.4.
When the temperature of the inlet of the second expansion engine 10.sub.2 is thus decreased to a temperature lower than 20.degree. K., the valve 5 is opened by the second preliminary operation, thereby cooling the line 11 of the fifth heat exchanger 9.sub.5, the valve 5 and the pipes connected thereto with the GHe thus cooled. Thus, the GHe is circulated from the return line 13 via the reservoir 4 to the inlet of the compressor 2 by opening the valve 8.
When the various units and components are thus cooled, the closed return valve 6 is then opened, the control valve 8 is closed, thereby circulating the GHe from the reservoir 4 from the return valve 6, the inlet 12' to outlet 12" of the line 12 and the inlet of the compressor 2. Thus, the third preliminary operation for cooling the return line has thus been completed. Thus, the cooled GHe from the fifth heat exchanger 9.sub.5 via the valve 5 is lowered at its temperature due to the isenthalpic expansion, and is stored as LHe in the reservoir 4. The J-T valve 5 is applied by the Joule-Thompson's effect known per se, to allow a temperature fall below a predetermined temperature and a temperature rise above a predetermined temperature. The boundary temperature (the Joule-Thompson coefficient is 0) between the temperature changes is called "an inversion temperature of the gas" , and the inversion temperature of the helium is 50.degree. K.
Since the conventional helium gas liquefying apparatus is thus constructed, the return valve 6 is heated through the valve 6 accommodating considerable amount of heat capacity, when the valve 6 is opened in the third preliminary operation. Further, the GHe thus heated is introduced into the line 12 of the fifth heat exchanger 9.sub.5, and the fifth heat exchanger 9.sub.5 is thus heated, with the result that the GHe is not cooled in the fifth heat exchanger 9.sub.5 but is, on the contrary, heated. Thus, the GHe thus heated is fed to the valve 5, and is heated to 70.degree. to 80.degree. K., thereby exceeding the inversion temperature of the GHe. The GHe is thus further heated, resulting in no production of the LHe even if the apparatus is started. Or, even if the GHe does not exceed the inversion temperature, the GHe thus heated to high temperature deteriorates the efficiency of the Joule-Thompson's effect as its drawback.